1. Field of Invention
The present invention is directed to scanning probe microscopy. More particularly, the present invention is directed to a method and apparatus for reducing the parachuting of a probe used in an atomic force microscope.
2. Description of Related Art
Scanning probe microscopes are used to make extremely high resolution measurements. An oscillating cantilever or probe is used to scan a sample surface and obtain information representative of the surface. Activity of the probe responsive to surface variations is monitored by a detection system. The detection system is typically an optical beam system. The height of the probe relative to the sample surface is then adjusted to maintain constant one or more probe operational parameters based on the monitored activity. Piezoelectric positioners are often used to adjust the height of the probe relative to the sample surface. Correlation of the adjustment amount in the Z direction versus the position of the probe along the surface in the X and Y directions is used to create a map of the surface.
Unfortunately, as the probe traverses the surface of the sample, the probe may not accurately follow the surface, particularly at abrupt drop-offs or ledges in the surface where the probe will tend to depart from and “parachute” over some portion of the surface. For example, when the probe reaches a drop off in the surface, the probe will only gradually lower itself again to the surface (the bottom of the drop-off) as the probe continues its scan, instead of immediately dropping to the surface below the ledge. This parachuting effect causes abrupt vertical drops that actually exist in the surface to be erroneously represented as gradual surface changes.
In particular, in current configurations of Tapping™ AFM scanning probe control, a probe is set to tap the surface at a constant oscillating amplitude. The change in the probe tapping amplitude, or other probe operational parameter, is used as a feedback error signal. As the cantilever probe traverses off an abrupt ledge or edge of a plateau, the cantilever leaves the surface, i.e., parachutes. Accordingly, the probe's oscillation amplitude will grow until the error is sufficiently large to cause a vertical (“Z”) motion piezo to respond and lower the cantilever towards the surface, reestablishing the pre-set amplitude, or other operational parameter. The duration of amplitude growth, and therefore error generation, can be quite long such that while the probe remains off the surface, surface features are passed by and not measured. To compensate for prolonged and slow error generation, and in an attempt to lower the probe to the surface more quickly so that surface features are not missed, the gain of the feedback error signal can be increased but this causes the tip to tap hard on the surface. Such hard tapping reduces error growth time and shortens the free parachuting state of the cantilever but causes other problems. Hard tapping causes the tip to impact the surface with great force which quickly results in damage to the tip and/or sample. Hard tapping is particularly damaging if the tip runs into an upward sloping feature where the probe slams into the surface and damages the tip and/or sample. Finally, these problems are exacerbated as scan rates increase.